Aa

L3 = 192.168.3.1 L3 = 192.168.3.2 L3 = 192.168.4.1 L3 = 192.168.4.2

Host-to-Host Packet Delivery

The steps to deliver an IP packet over a routed network are similar to the steps to send a letter through a mail delivery service. The key is to know the sender's source address and the destination address of the data. There are a number of steps involved in delivering an IP packet over a routed network. The next several figures give you a graphical understanding of the process.

Note from earlier discussion that an IP host determines the network it belongs to from its IP address and subnet mask. The host will send any packet that is not destined for the local IP network to the default gateway. The default gateway is the address of the local router, which must be configured on hosts (PCs, servers, and so on).

In Figure 4-31, host 192.168.3.1 has application data that it wants to send to host 192.168.4.2. The application selects a User Datagram Protocol (UDP) as the transport for this data, indicating that the reliability of delivery is unimportant for this application or handled by application.

Figure 4-31 Application Sends Data to Transport Stack

Figure 4-31 Application Sends Data to Transport Stack

Because it is not necessary to set up a session, the application can start sending data. UDP prepends a UDP header and passes the protocol data unit (PDU) to IP (Layer 3) with an instruction to send the PDU to 192.168.4.2. IP encapsulates the PDU in a Layer 3 packet and passes it to Layer 2. This process is illustrated in Figure 4-32.

This example differs from previous examples of sending packets (now called frames) at Layer 2 because the two hosts are on different segments: 192.168.3.0/24 and 192.168.4.0/24. Because the host is configured with an IP address and a subnet mask, it understands that 192.168.4.0 is on a different network. Because it does not know how to get to this network, it must send the frame to its default gateway, where the frame can be forwarded. If the host does not have a Layer 2 mapping for the default gateway, the host uses the standard ARP process to obtain the mapping for the router. Figure 4-33 shows this process.

Figure 4-32 Transport Sends Data to IP Stack ri

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